Presence and data labels

ABSTRACT

A system for detecting presence of and for communicating with an electronic coded label. The coded label includes a presence signaling antenna, a label circuit and a data communications antenna. The system includes presence detection means for creating a presence detection electromagnetic field and data communications means for creating an interrogation electromagnetic field. The label circuit is adapted to operate in a first mode when the electromagnetic field to which the label is exposed is at a first power level, said first mode being that of a linear circuit with an impedance dependent upon frequency. The label circuit is adapted to operate in a second mode when the electromagnetic field to which the label is exposed is at a second power level, said second mode being that of a non-linear circuit with behavior dependent upon time.

BACKGROUND OF THE INVENTION

The present invention relates to a system for both detection of thepresence of, and for remote identification of or telemetry of data to orfrom, objects using electronically interrogatable coded labels. Inparticular the invention relates to a system for automated detection ofthe presence of labels attached to merchandise, and automated input toor extraction of data from those labels.

An electronic label, the presence of which may be detected by fielddisturbance principles without the particular label being thenidentified, but which is also capable of carrying electronically codeddata, may be attached to an article of merchandise.

An electronic system called a presence detection interrogator andcontaining an antenna which creates an electromagnetic field in an exitregion from a shop is able to detect the occurrence of an article taggedwith one of the labels passing through the field. When the presence ofthe label is so detected an alarm is raised, as it is presumed that thearticle has not been paid for. However, if the article has been properlypaid for, the label can at the point of sale be removed or have itspresence indicating function disabled, so that no alarm is raised whenthat label passes through a scanned area.

As well as having a capacity to signal its presence through fielddisturbance, the label may be electronically encoded with data which maybe read either by presence detection means or data communications means.The presence detection means may include a presence detection signalgenerator, a presence detection antenna for creating a presencedetection electromagnetic field, and an analyser of impedance associatedwith the presence detection antenna. The data communications means mayinclude a generator of an interrogation signal, an interrogation antennafor creating an interrogation electromagnetic field, a data receiverantenna, and a receiver and decoder of data communication signals. Thesignals received and decoded by the receiver and decoder may bepresented as an output signal to a data processing system.

Although the present invention is herein described with reference to amerchandising operation, it is to be appreciated that it is not therebylimited to such application. Thus the presence and data label can beapplied to object identification operations generally, and has obvioususes in libraries and in document control environments.

A simplified diagram of the system is shown in FIG. 1A wherein alabelled item 1 carries an electronic label 2, the presence of which(assuming that function is not disabled) can be detected by a presencesensing antenna 3 connected to a label presence detector 4 andproviding, when a label is detected, a label presence output signal LPO.Data which is present within label 2 may be extracted therefrom by adata interrogation antenna (DIA) 5 which is connected to a datacommunications interrogator (DCI) 6. Referring to FIG. 1B when it isdesired that label 2 should produce no signal from presence detector 4(ie. item 1 has been paid for), label 2 is placed over a disabling plate7 which upon receipt of a disabling input signal DIS to a disablingsignal generator 8, generates a disabling electromagnetic field. Thelatter changes the nature of a circuit contained within label 2 in amanner to be described herein.

The disabling system may also contain means to change or to read thedata within label 2 and also to provide a programming signal to label 2so that the data may be changed. In this way reading of the label's datamay be performed to signal to or to confirm that the presence signallingsection of label 2 has been legitimately disabled.

In the construction of presence detection and data carrying labels, animportant design parameter is the physical size of the antenna sectionof the label, as both presence detection range and data interrogationrange depend upon the size of that antenna. It is therefore desirablethat the antenna for either of these operations be made as large aspossible. One way in which this may be achieved is to make a singleantenna connected to appropriate circuit elements fulfill both of thepresence detection and data communication functions. Another approach isthe employment of separate antennas, together with ensuring that theirsize and relative positioning is such as to permit a maximum antennasize consistent with any necessary separation of the functions ofpresence detection and data communication.

In the achievement of sufficient range for both data communicationoperations and presence detection operations, note should be taken ofelectromagnetic compatibility regulations which limit the value ofelectromagnetic field which may be legally established for each of thesetwo functions. In many jurisdictions a CISPR quasi-peak detector is theinstrument specified for making measurements of fields which are to becompared against regulations. Because the response of that measuringinstrument depends on a complex and in some aspects non-linear way uponboth the field amplitude and its time dependence, it is appropriate inthe achievement of long range for both of the functions fulfilled by thelabel, that the electromagnetic signals which perform these functions,and the response of the label to those signals, should be shaped toachieve the largest possible range achievable within the regulations.This requirement needs to be taken into account in both the design ofthe label and the design of possibly combined or possibly separateinterrogator systems for functions of the label.

In the practical operation of systems as described herein it may benecessary to disable the presence detection section of a label toindicate that the attached item has been paid for. One method by meansof which such disabling may be carried out is to subject the presencesignalling section of the label to an intense electromagnetic field at afrequency of resonance of that section so as to produce failure of oneof the presence signalling circuit elements. It is normally desired thatthe data communication section of the labels survive this operation, andthe latter section should therefore be designed accordingly.

In the design of systems which have the capability for both presencedetection and data transmission, it is necessary to take into accountthe frequencies normally used for exploitation of these functions. Inrespect of presence detection, resonance of the label within thefrequency band 8.0-8.4 MHz is frequently employed, but other frequenciesseem to be equally useful under the regulations, although spectraloccupancy by other users may be a relevant consideration. For datacommunication, the frequencies 27.12 MHz, 13.56 MHz and 6.78 MHz areuseful in that electromagnetic compatibility regulations allow, within anarrow band surrounding these frequencies, transmissions ofsignificantly greater amplitude (by 20 dB and sometimes by a muchgreater amount) than in the immediately adjacent bands. The greatestrange can be achieved if it can be arranged that the label antenna inthe data communication mode of the label is resonant at one or other ofthese frequencies.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided asystem for detecting presence of and for communicating with one or moreelectronic coded labels, the or each coded label including a presencesignalling antenna, a label circuit and a data communications antenna,said system including:

presence detection means for creating a presence detectionelectromagnetic field;

data communications means for creating an interrogation electromagneticfield;

wherein the or each label circuit is adapted to operate in a first modewhen the electromagnetic field to which the associated label is exposedis at a first power level, said first mode being that of a linearcircuit with an impedance dependent upon frequency, and the or eachlabel circuit is adapted to operate in a second mode when theelectromagnetic field to which the associated label is exposed is at asecond power level, said second mode being that of a non-linear circuitwith behaviour dependent upon time.

According to a further aspect of the present invention there is providedan electronic coded label for use with a system for detecting presencethereof and for communicating therewith, said label including a presencesignalling antenna, a label circuit and a data communications antenna,wherein said label circuit is adapted to operate in a first mode when anelectromagnetic field to which said label is exposed by said system isat a first power level, said first mode being that of a linear circuitwith an impedance dependent upon frequency, and said label circuit isadapted to operate in a second mode when the electromagnetic field towhich said label is exposed by said system is at a second power level,said second mode being that of a non-linear circuit with behaviourdependent upon time.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Preferred embodiments of the invention will now be described withreference to the accompanying drawings wherein:

FIG. 1A shows major sub units of a system performing both presencedetection and data communication to and from an electronic label;

FIG. 1B shows a system for disabling the presence signalling section ofa label;

FIG. 2 is a block diagram of one variety of presence detecting and datacommunicating label;

FIG. 3 is a circuit diagram showing significant elements of a datacommunicating microcircuit;

FIG. 4 shows resonant curves of circuits within a presence detecting anddata communicating label;

FIG. 5 is a circuit diagram showing a different form of presencedetecting and data communicating label;

FIG. 6 shows an arrangement of antennae in a presence detecting and datacommunicating label;

FIG. 7 shows resonant curves in another form of presence detecting anddata communicating label;

FIG. 8 shows circuit elements useful in another form of presencedetecting and data communication label;

FIG. 9 shows an arrangement of printed windings useful in the label ofFIG. 8;

FIG. 10 shows a block diagram of a form of controller and modulatorcircuit;

FIG. 11 shows a relationship between a label and a presence detector forpresence detection;

FIG. 12 shows functional blocks present in one form of a presencedetector;

FIG. 13 shows an antenna configuration useful for label presencedetection;

FIG. 14 shows an antenna configuration useful for data communicationwith a label; and

FIG. 15 shows functional blocks present in one form of datacommunication interrogator.

In the system of which an outline is shown in FIG. 1A, labelled item 1carries presence and data label 2 which is capable of modifying thefield created by presence sensing antenna 3. The latter may receivesignals from and may transmit signals to presence detector 4. Presenceof label 2 is indicated by means of a high quality factor resonantcircuit which, when it is exposed to the field of presence sensingantenna 3, produces a frequency sensitive modification of the impedanceof antenna 3. The change in impedance of antenna 3 is signalled bypresence detector 4, which explores over a frequency range within whichlabel resonance is expected to occur, the impedance of presence sensingantenna 3.

When an impedance change in presence sensing antenna 3 greater than athreshold value is observed, a label presence output signal LPO emergesfrom presence detector 4.

As an alternative to detecting the changed impedance of a singleantenna, label presence may be sensed by detecting over a narrowfrequency band, a change in coupling between a plurality of antennae orantenna parts forming with the presence detector 4 the presence sensingsystem. In such a case the antennae or antenna parts in the presencesensing system may be arranged to be substantially uncoupled in theabsence of a label, and the label may provide some frequency sensitivecoupling therebetween.

As indicated in FIG. 1A label 2 has, as well as the capacity ofsignalling its presence through disturbance of the field created bypresence sensing antenna 3, a capacity to communicate, over anelectromagnetic coupling link between label 2 and a data interrogatorantenna (DIA) 5, with a data communications interrogator (DCI) 6. Label2 may contain no energy source and receive energy for operation of itsdata communication function from the electromagnetic field created byDIA 5. As well as providing energy to label 2, the signal from DIA 5 maycontain information which may be programmed into label 2. Label 2 mayalso emit a reply signal which may be received by DCI 6 via DIA 5.

As is indicated in FIG. 1B, the system may also contain label presencedisabling plate 7 which receives a label disabling signal from disablingsignal generator 8, the later signal being generated when disablingsignal generator 8 receives a disabling input signal DIS. For the labeldisabling operation to occur, label 2 is placed close to label disablingplate 7 which generates a label disabling electromagnetic field. Thelabel disabling field may take the form of a strong electromagneticfield at the resonance frequency of the tuned circuit which provides thelabel presence signalling function.

At approximately the same time as the label disabling field isgenerated, the data section of label 2 may receive via a furtherelectromagnetic field, similar to that generated by DIA 5, a signal,which may be coded, indicating that label disabling is to be, or hasbeen, performed, and the data within label 2 may change in response toreceipt of this signal.

A block diagram of a basic form of presence and data label 2 is shown inFIG. 2. Presence and data label 2 contains a tuned circuit includinginductor 9, a first capacitor 10, a second (optional) capacitor 11 and adata communications microcircuit (DCM) 12 which is connected atterminals A and B to the tuned circuit provided by inductor 9 andcapacitors 10 and 11. The input impedance of DCM 12 is under certainconditions, expected to be reasonably represented by a capacitance inparallel with a resistance, but the validity of this representation andthe values of both of these elements depend upon the signal levelgenerated within the overall tuned circuit, and upon operations whichtake place within DCM 12.

Some internal circuits of one preferred embodiment of DCM 12 are shownin FIG. 3. DCM 12 may contain a capacitor 13 which makes a contributionto establishing a resonant frequency of the circuit including inductor 9at both low and high power. A bridge rectifier circuit 14 can providesupply voltages VDD and VSS to a controller and modulator (CAM) circuit15.

Signals PGC and NGC emerge from CAM circuit 15 to control transmissiongate 16 which when closed introduces, in series, capacitors 17 and 18.The latter are placed across the input terminals marked A and B andtherefore modify the resonance frequency of the tuned circuit includinginductor 9. Resistors 19 and 20, which may be merely representations ofthe on-resistance of the transmission gate 16, have the effect that thequality factor of the aforementioned resonance is reduced at the sametime as the frequency is lowered. Capacitor 21 is simply a reservoircapacitor for bridge rectifier 14.

It will be clear from the above description that label 2 acts as alinear frequency dependent device in a presence detection mode, and actsas a non linear time dependent device in a data communications mode. Thefactors which cause it to change its mode of operation may include achange of frequency or a change of amplitude of the electromagneticfield to which label 2 is exposed. In both cases it is the amplitude ofthe signals internal to label 2, in relation to threshold voltageswithin semiconductor junctions or in field effect transistors, whichdetermine in which mode label 2 operates from time to time.

A notable feature of the invention is that both the presence detectionsignal and the data communications signal cause currents to flow in theentire label circuit, but the amount of the current, and itsdistribution in amplitude within the various parts of the label circuit,are each dependent upon both the frequency and the power level of theexcitation electromagnetic field.

Many of the design features and operations of CAM circuit 15, includingits capacity to receive programming information through modulation ofthe radio frequency signal which excites the label; its capacity tostore in non-volatile memory that information and when appropriate toproduce modulation of transmission gate 16 in the form of binaryfrequency shift coding of a sub-carrier frequency generated from anoscillator within the controller and modulator section; are described inapplicant's PCT AU/92 000143 and PCT AU/92 000477 the disclosures ofwhich are incorporated herein by cross reference.

Adding to those ideas, CAM circuit 15 may incorporate in addition a timedelay, the function of which is to be described herein, between theapplication of the VDD and VSS supply voltages and the occurrence of anymodulation via transmission gate 16, Prior to the occurrence of suchmodulation, transmission gate 16 is constrained to remain open.

The effect of operation of DCM 12 on the resonance curve-of the tunedcircuit-it including inductor 9 are shown in FIG. 4 wherein themagnitude of the impedance of that tuned circuit is plotted againstfrequency for various conditions.

The narrow resonance curve A of large amplitude and small bandwidthrepresents the condition when the tuned circuit is operating at a powerlevel too low for the development of any significant rectified voltagevia bridge rectifier 14, and also approximately the condition when avoltage sufficient to operate CAM circuit 15 has been developed, but asufficient time delay has not elapsed for transmission gate 16 to haveclosed or modulation to have commenced. In consequence, significantloading of the tuned circuit including inductor 9 may not yet haveoccurred, and the effective input capacitance of bridge rectifier 14 mayremain approximately at the value it has for small signals. Thus boththe high quality factor and the resonant frequency of the resonanceincluding inductor 9 will be maintained approximately at their smallsignal values, and that resonance continues to provide the previouslydiscussed field disturbance of the impedance of the presence sensingantenna 3 upon which label presence detection is based.

Also shown in FIG. 4 are resonance curves B and C of lesser amplitudeand lower resonance frequency. Curves B and C show the conditionsobtained when a higher amplitude and longer duration excitation signalis applied so that supply voltages VDD and VSS, sufficient for normaloperation of CAM circuit 15, are developed, but for which the time delaywhich may be built into that circuit has elapsed so that modulation oftransmission gate 16 has commenced.

Curve B having the larger amplitude and higher resonant frequencyrepresents the condition when transmission gate 16 is open, and curve Chaving the lower amplitude and lower resonance frequency represents thecondition when transmission gate 16 is closed.

In the presence detection mode of label 2, it is appropriate that it beexposed to a field which explores, over the frequency range F_(pl) toF_(ph) shown in FIG. 4, the nature of non-operating resonance curve A.As will be described herein, this exploration may preferably be via asignal which is swept rapidly at an appropriately controlled rate overthat band.

In one preferred embodiment, for the data communication mode ofoperation of label 2, the label may be interrogated by a substantiallycontinuous wave signal at a data interrogation frequency F_(di)positioned as shown in FIG. 4. During that interrogation, the tunedcircuit resonance switches between the two curves B and C shown in FIG.4, with the result that the signal amplitude in inductor 9 varies, inboth amplitude and phase, between two values which are defined by theoperating points X and Y shown in FIG. 4. The amplitude and phasevarying oscillation appears as a reply signal sub-carrier generated bymodulation of the interrogation and energising signal at the frequencyF_(di), and appears as sidebands of the data interrogation frequency.The actual reply signal is modulated on to that sub-carrier, as theresult of CAM circuit 15 varying the pattern with which transmissiongate 16 opens and closes.

In another preferred embodiment the positions of resonance curves B andC for the switch open and the switch closed positions are both shiftedto the right but by different amounts so that points X and Y have thesame amplitude. The benefit of this arrangement is that the amplitude ofexcitation of the data communications circuit is similar for both theswitch open and switch closed positions. Because of the differentphases, relative to the excitation signal in the data interrogator, ofthe current in inductor 9 for the two conditions, a replyelectromagnetic field is still generated.

The preferred embodiment of the invention described so far has thebenefit that a single inductor antenna 9 is used for both the presencedetection and data communication function of the label, and that theantenna may conveniently be shaped to occupy almost all of the area ofthe label, with the result that the sensitivity of the label to both thepresence detecting field and the data communication field is maximised.

In this embodiment a number of techniques are available for changing thenature of the label to disable, if desired, its presence detectionfunction. In a preferred embodiment this can be accomplished by theremoval of capacitor 11, possibly by a punching operation, withcapacitors 10, and 11, and the input capacitance of DCM 12 beingproportioned so that the result of removal of capacitor 11 is asubstantial rightward shift of the three resonance curves A, B and Cshown in FIG. 4.

The result of this shift may be that the high quality factor resonancewhich is observed at low power and prior to the operation of the timedelay built into DCM 12 is shifted well to the right of the frequencyspan over which the impedance characteristics of the label are exploredin the presence detection mode. Although resonance curves B and Ccharacteristic of label operation in the data communications mode, willalso be shifted to the right, they may still be wellpositioned inrelation to the data communication interrogation frequency F_(di),perhaps so that the point of intersection of those curves takes place atthe data communications interrogation frequency F_(di), so that goodextraction of power from the communication field, and substantialmodulation of the oscillation as transmission gate 16 opens and closes,if not in amplitude, then at least in phase, can occur. Two-way datacommunication to and from DCM 12 is still then eminently possiblewithout undue loss of sensitivity. Although these resonance curves maythen lie within the frequency range explored by the presence detector,the amplitude and quality factor of both those resonances will be toolow to activate the presence sensing mechanism provided by presencesensing antenna 3 and presence detector 4.

In an alternative embodiment of the invention which allows deactivationof the presence detecting mechanism to be safely accomplished bynon-contact means is shown in FIG. 5. In this embodiment, presencedetection is accomplished through establishment of a high quality factorresonance at an appropriate frequency between inductor 22 and capacitors23 and 24, while data communications operations are accomplished atgenerally different but appropriate frequency through the action ofinductor 25, capacitor 26 and DCM 12. So that large amplitude signalsmay be applied to one circuit without introducing excessive amplitudesignals in the other, the mutual inductance M between inductors 22 and25 is kept small, and may be set to zero with an appropriate design. Anarrangement of conductors which shows the relationship between atwo-turn planar winding for inductor 22 and a single-turn planar windingfor inductor 25, in which arrangement the overlap may be adjusted toachieve zero mutual inductance, is shown in FIG. 6. Node labels C, D, Eand F in FIG. 5 correspond to similar labels in FIG. 6.

In this embodiment capacitor 24 shown in FIG. 5 can be made with thindielectric having a low breakdown voltage, while capacitor 23 is a lowloss capacitor which is relatively robust and is not subject to suchbreakdown. Capacitor 24 subject to breakdown may be made significantlysmaller than capacitor 23, so that different manufacturing tolerancesmay be applied to each in case the manufacture of a capacitor with lowbreakdown voltage accidentally or incidentally involves a greatervariation in capacitance value.

In this embodiment, disabling the presence detection function may beachieved by applying through label disabling plate 7 a large oscillatingmagnetic field at the resonance frequency formed by inductor 22 andcapacitors 23 and 24, with the result that voltages which exceed thebreakdown voltage of capacitor 24 are generated, and capacitor 24 breaksdown to become a short circuit. The result is the resonance of thattuned circuit is extinguished despite the intact nature of inductor 22and capacitor 23.

In another embodiment capacitors 23 and 24 shown in FIG. 8 might be inseries. In that embodiment disabling of the presence detection functioncan be achieved, while leaving the data communications function intact,albeit at a possibly different operating frequency, by placing a shortcircuit in parallel with one of the capacitors 23, 24. It is alsopossible to disable the presence indicating function by the introductionof an open circuit within a part of the presence signalling resonantcircuit.

In the disabling operation it is important that damaging voltages arenot placed across DCM 12. This may be accomplished by firstly insuringthat mutual inductance between inductors 22 and 25 is small, andsecondly that the resonant frequency of inductor 25 and the resultingcapacitance across terminals E and F is substantially different from theresonant frequency of the presence detection section of the label.

A substantial separation of the relevant frequencies and the resonantcurves applying to the circuit of FIG. 5 is shown in FIG. 7. Thepresence detection circuit has a resonant frequency shown as F_(pb)associated with curve A in FIG. 7, and presence detection is carried outwithin the frequency band F_(pl) to F_(ph). The data interrogationfrequency is shown as F_(di) and may for example be 13.5 MHz, whereaspresence detection might be carried out in the vicinity of 8.2 MHz. Theswitch-open and switch-closed resonance curves formed by inductor 25 andcapacitors connected thereto are marked B and C respectively in FIG. 7.It may be ascertained, by studying these resonance curves in relation tothe data interrogation frequency, that good transfer of power to thelabel from the data interrogation signal may be achieved at both openand closed positions of the switch within DCM 12, and that there will besignificant phase shift in the oscillation produced at inductor 25between the switch-open and switch-closed positions, although there maybe little or no amplitude modulation. Thus good power transfer to thedata communication section of the label and generation of a strong replyare both assured.

A further embodiment of the invention which shows how damaging voltagesgenerated during the label disabling phase may be kept from reaching DCM12 is shown in FIG. 8. FIG. 8 differs from that of FIG. 5 through theaddition of series-connected components capacitor 27 and inductor 28which are resonant at the frequency F_(pb) at which the presencedetection function of the label is disabled. The fact that inductor 25and capacitances connected thereto are both resonant at that frequency,and the branch containing capacitor 27 and inductor 28 has a lowimpedance at that frequency, combine to reduce the voltage applied toDCM 12 in that operation to a low value.

A suitable arrangement of windings on a flat label to accomplish thefunction of FIG. 8 is shown in FIG. 9. Again node labels A, B, C, D, E,and F introduced in FIG. 8 identify corresponding nodes in FIG. 9. Theoverlap between inductor 25, here represented for simplicity as a singleturn, and inductor 22 is adjusted to achieve approximately zero mutualinductance. Inductor 28 is realised as a figure-eight configuration, sothat it has firstly no mutual coupling to inductor 22, secondly nomutual coupling to inductor 25, and thirdly no net excitation from auniform externally applied field.

Further measures by means of which DCM 12 may be protected fromovervoltage are shown in FIG. 10. CAM circuit 15 may contain a decodermemory and modulator (DMM) circuit 29 which is responsible for receivinginformation from the interrogation field, storing in memory andgenerating a reply signal through modulation of the previously mentionedtransmission gate. Excessive voltage developed between the VDD and VSSsupply rails is prevented through operation of over-voltage limiter(OVL) circuit 30 which provides a sharp increase in conductance when thevoltage applied to its terminals exceeds an upper limiting thresholdvoltage.

At much lower power levels, operations within DCM 12 are inhibited untilthe supply voltage exceeds a preset threshold provided by voltagethreshold detector (VTD) 31. Such operations do not, however, commenceas soon as the threshold voltages are exceeded. The output of VTD 31 isapplied to time delay circuit (TDC) 32 which prevents any significantoperation, other than operation of OVL 30, and in particular within DMMcircuit 29, until in DCM 12 a time delay, set within TDM 32, haselapsed.

The advantage of including these elements is that the high qualityfactor resonance of the presence detecting circuit is preserved for atime sufficient for that resonance to be accurately probed by thepresence detection field, without that resonance being disturbed byoperation of other parts of the label. Of course such independence isalso aided by the above described techniques for establishing zerocoupling between the presence detection and data communications sectionof the label.

Some aspects of the relationship between the label presence sensingantenna system and presence detector 4 are shown in FIG. 11. Here thepossibility of presence sensing antenna 3 consisting of two coils 33 and34 which are symmetrically excited by presence detector 4 but areuncoupled from receiving antenna 35 is shown. Introduction of label 2into the antenna system can create, over the narrow band of resonance ofthe presence detection system of label 2, coupling where none hasexisted before.

As an alternative, presence sensing antenna 3 may consist simply ofinductor 35 without inductors 33 or 34 being present. In that case thepresence of label 2 provides a disturbance to the impedance of inductor35 seen by presence detector 4.

A block diagram of a preferred embodiment of presence detector 4 isshown in FIG. 12. Although this detector is configured for detection ofimpedance anomalies in a single presence sensing antenna 3, obviousmodifications allow it to be applied to detection of label inducedanomalies in the coupling between a plurality of presence sensingantennae.

Signals for presence sensing are generated in a frequency selectableoscillator (FSO) 36. FSO 36 might smoothly sweep over the frequencyrange F_(pl) to F_(ph) for which the label resonances are to be searchedat an appropriate rate, to be discussed herein, or might interrogate forappropriately short periods at a pseudo-randomly specified set ofinterrogation frequencies within that band. The output of FSO 36 may berouted via transmitter amplifier gate (TAG) 37 through transmitter poweramplifier (TPA) 38 and signal separation system (SSS) 39 to labelpresence antenna (LPA) system 40 which consists of inductor 35 tuned bycapacitor 41 and damped by loading resistor 42. The objective of thelatter components is to provide a resonance frequency centred in theband F_(pl) to F_(ph) to be searched and with a bandwidth significantlyexceeding the width of that band. As LPA system 40 is normally somedistance from the presence detector 4 (not shown in FIG. 12), LPA 40 ismatched to a transmission line 42A connecting LPA 40 to presencedetector 4.

Within presence detector 4, SSS 39 may consist of a directional coupleror a directional bridge. In either case the reference arm is terminatedin signal separation termination (SST) 43 which is intended to consistof a tuned circuit whose resonance curve closely matches in shape thatof LPA system 40. Controllable elements voltage controlled capacitor(VCC) 44 and voltage controlled resistor (VCR) 45 are adjusted asdescribed herein to achieve this result.

The unbalanced signal emerging from SSS 39 is amplified in receiverpreamplifier (RPA) 46 the output of which becomes the input signal to apair of balanced mixers 47A and 47B which receive as their localoscillator signals a version of the signal emerging from FSO 36, thephase of this oscillation having been in one case retarded 90 degrees byquadrature phase shifter 49. The output signals from balanced mixers 47Aand 47B are amplified in base band amplifiers 48A and 48B, converted todigital form in analogue to digital converters 50A and 50B, andpresented to microcontroller 52 as digital representations of the realand imaginary parts of the unbalance between LPA system 40 and SST 43 ofSSS 39. Microcontroller 52 is responsible for generating control signalsto FSO 36, TAG 37, and SST 43.

In operation, when the system is first turned on and also when it may bereasonably assumed that no labels are in the field of LPA 40, controlledelements VCC 44 and VCR 45 and if necessary inductor 51 within SST 43are adjusted to minimise mean square error of the unbalanced signalemerging from SSS 39 over the band for which LPA impedance is beingexplored. Special treatment is also given to signals, which may bedetected by RPA 46 and circuits following, during the period in whichTAG 37 is closed. Such signals are deemed to have arisen from extraneoustransmissions from other users of the spectrum, and are expected to beirrelevant, and not representative of label impedance, with the resultthat the data reaching microcontroller 52 in the vicinity of suchperiods is discarded.

When a label 2 is exposed to the field of LPA 40, the profile ofresidual signals emerging from SSS 39 across the band being exploredwill be disrupted. Provided there are no extraneous transmissions, thisdisruption of profile will be detectable by microcontroller 52 and willif of sufficient amplitude cause the issuing of label presence outputsignal LPO as shown in FIG. 12.

A configuration of antenna suitable either for presence detector 4 orDCI 6 at significant range is shown in FIG. 13. The antenna consists oftwo separate parts 53 and 54 each with a feed point in the gap shown atthe centre. The antenna parts 53, 54 are intended to be linked inparallel via equal length cables, connected to the feed points. Thelatter are marked as G and H in part 53 and I and J in part 54, withpolarity for the connection also being indicated. Customers withlabelled objects are intended to travel between antenna parts 53, 54 inthe direction shown by arrow A.

Although each antenna part 53, 54 is shown in a horizontal disposition,that is with the longer dimension in the direction of travel, it can berotated through 90 degrees so that the longer dimension is vertical andorthogonal to the direction of travel.

The current distribution in each antenna part 53, 54 is in theorientation shown vertically across the feed point, with equal andopposite currents returning to the feed point via horizontal sectionsand outer vertical sections. With such currents, it can be seen thateach part 53, 54 can be regarded as a pair of magnetic loops displacedhorizontally, and with equal and opposite excitation.

An advantage of the structure is that for a point at the centre, thehorizontal components of the magnetic field caused by selected pairs ofthe vertical sections are of the same phase and in the same directionand therefore reinforce, whereas for points external to the structureand distant from it the magnetic field components from the same pairsapproximately cancel. The cancellation is not complete, because eachmember of a pair has a different distance to the distant field point.However, pairs of cancelling currents within the one antenna partthemselves approximately cancel in pairs, and moreover the field fromthe entire distribution of current in one antenna part approximatelycancels at a distant external point the field from the distribution ofcurrents in the other antenna part.

Viewed in this way, each antenna part 53, 54 is composed of twooppositely phased magnetic dipoles, thus forming a magnetic quadrupole,and the two oppositely directed quadrupoles formed by the two parts forma magnetic octopole. It is a property of that magnetic octopole that atsufficient distance the residual external magnetic field diminishes asthe fifth power of distance. This cancellation relation is particularlyadvantageous for creating, within the structure a strong magnetic field,while the magnetic field at the distance of 10 meters at whichelectromagnetic compatibility regulations are commonly enforced, is verymuch less.

As indicated above this antenna structure may be used to advantage withpresence detector section, or the data communication section of theoverall system.

An alternative antenna configuration which is particularly useful in thedata communication section of the overall system is shown in FIG. 14.The latter antenna is useful in the hand-held, so-called reading-wandtype of data communication antenna for communicating with the datacommunication section of the label. The reading-wand antenna takes theform of a rod 55 on the ends of which are placed oppositely sensed coilwindings 56 and 57 connected in series. Windings 56 and 57 are tuned, inthis case to parallel resonance, through capacitor 58 and loaded byresistor 59 to create an appropriate resonant impedance well matched totransmission line 60 which connects the readingwand antenna to DCI 6 andto a signal separation system within that unit.

An advantage of this form of antenna structure is that whereas the fieldat a position of label 2 placed close to one end of rod 55 is strong andapproximately dipolar, at a large distance the oppositely directed andalmost equal fields created by windings 56 and 57 generate a quadrupolefield which diminishes as the inverse fourth power of distance from thecentre of rod 55, with the result that the ratio of the field availableat the position of label 2 to the field which is allowed to be generatedat the electromagnetic compatibility enforcement distance, commonly 10meters, is enhanced over that of a simple dipole.

A block diagram of a preferred form of DCI 6 suitable for the datacommunication section of the overall system is shown in FIG. 15. In thissystem signals for energising and communication to label 2 are generatedwithin frequency selectable oscillator (FSO) 61, are gated on and off bytransmitter enable gate (TEG) 62 and amplified in transmitter poweramplifier (TPA) 63 after which they pass through signal separationsystem (SSS) 64 to data interrogation antenna (DIA) 65 which creates thefield for energising, and also provides the function of communicating toand receiving a reply from label 2, the latter not being shown in FIG.15.

In operation, signals from label 2 via DIA 65 are passed back to SSS 64.SSS 64 can take the form of a directional coupler or a directionalbridge. In either case it may be a four-port device terminated in oneport in signal separation termination (SST) 66, the impedancecharacteristics of which may be made to resemble those of DIA 65 so thatin the absence of any reply signals from label 2, there is approximatebalance between the signals coupled from TPA 63 to the so farundiscussed port of SSS 64 by reflection from the two terminationsprovided by DIA 65 and SST 66.

The output signal from SSS 64 consists of two components namely (i)uncancelled reflections from terminations DIA 65 and SST 66, and (ii) aportion of the label reply signal detected by DIA 65. Those signals areamplified in receiver preamplifier (RPA) 67 and passed to balanced mixer68 which receives as its local oscillator signal the originallygenerated oscillation from FSO 61. The output from balanced mixer 68 isamplified in receiving baseband amplifier 69 and is sampled in analogueto digital converter 70, the samples being passed to microcontroller 71,wherein the sample values of the reply are decoded, with the decodedsignal DSO emerging from microcontroller 71. Microcontroller 71 alsogenerates frequency selection signals for FSO 61. Control signals forTEG 62, which is used either in transmitting data to the label throughmodulation of the transmission thereto, or is used to allow the datasignal interrogator to operate in a pulsed mode wherein transmissionsoccur for a period long enough for the extraction of a complete replyfrom the interrogator, the transmissions ceasing for a period longenough for the sampled reply passed to microcontroller 71 to be decoded.

Microcontroller 71 also generates control signals for SST 66. Thefunction of those control signals is to allow adaptive adjustment of SST66 so that uncancelled reflections being passed from SSS 64 to RPA 67are minimised.

In many respects the structure described can be regarded as astraightforward homodyne receiver. In such receivers the phaserelationship between the oscillator signal used as a local oscillatorfor the balanced mixer and the carrier signal which returns from thelabel must be controlled so that they are not in quadrature. For thispurpose a controllable phase shifter 72 is inserted in the localoscillator line to balanced mixer 68 and may receive control signalsfrom microcontroller 71 to ensure that this quadrature relation does notoccur.

In the preferred embodiment of a DCI as shown in FIG. 15, FSO 61 may beswept or switched rapidly in a pseudo-random manner through a set ofinterrogation frequencies all falling reasonably well within the passband of DIA 65, in which the quality factor may be controlled throughadded damping to achieve a suitable bandwidth. In the switching process,the dwell time upon any one of the frequencies should be short comparedto the rise time of the intermediate frequency amplifier of the CISPRquasi-peak detector instrument used in the specification ofelectromagnetic compatibility regulations, but should be long comparedwith the small rise time of the tuned circuit in DIA 65 and the tunedcircuit involving the label data communication antenna, both of whichwill be of considerably greater bandwidth than the previously mentionedintermediate frequency amplifier of a quasi peak detector instrument.The dwell time should be short in relation to the sub-carrier frequencygenerated within the label for the purpose of carrying its reply.

With interrogation signals generated in this way, the field amplitudeindicated by the quasi peak detector will be significantly less than thepeak field amplitude, which should be kept high in order to obtain anadequate energising level for the label. The modulation of theenergising frequency should not, however, occur to a significant degreeat the output of balanced mixer 68, as both the reply signal carrier andthe local oscillator fed to the input ports of mixer 68 will be trackingin frequency. This type of interrogator is particularly useful when nospecial bands allowing an elevated interrogation field level above thesurrounding bands are available.

In the operation of a presence detector as shown in FIG. 12, thefrequency selectable oscillator may be swept rapidly over the band whichis searched for the high quality factor resonance, or may be switched ina pseudo-random manner between a number of frequencies which cover thatband. In either case, the amount of time spent at any one frequency, orspent in sweeping through a bandwidth equal to the bandwidth of theintermediate frequency amplifier of the

CISPR quasi-peak detector instrument, should be small in relation to the1 ms charging time which is specified in the CISPR standard for thedetector section of that instrument, while at the same time, that timespent should be long in relation to the rise time of an oscillationwithin the presence signalling tuned circuit of the label discussedherein.

In this way the signals used for presence detection have an adequatetime to establish significant amplitude oscillations within the label,but insufficient time to establish significant charging of the reservoircapacitor of the detector in a quasi-peak detector receiver. The outputof such receiver is therefore significantly less than would be indicatedby a continuous wave oscillation of the same amplitude, as is beinggenerated by the presence detector just described. The result is thatwhen a swept frequency, or frequency hopping technique is used,significantly stronger amplitude signals may be generated by thepresence detector, without violating electromagnetic compatibilityregulations, than would be permitted if a continuous wave oscillation ora slow sweep, that can develop full amplitude within the intermediatefrequency amplifier and detection of a quasi-peak detector, were to beused.

It will be appreciated that various alterations, modifications and/oradditions may be introduced into the constructions and arrangements ofparts previously described without departing from the spirit or ambientof the present invention.

What is claimed is:
 1. A system for detecting presence of and forcommunicating with one or more electronic coded labels, the or eachcoded label including a presence signalling antenna, a label circuit anda data communications antenna, said system including:presence detectionmeans for creating a presence detection electromagnetic field; datacommunications means for creating an interrogation electromagneticfield; wherein the or each label circuit is adapted to operate in afirst mode when the electromagnetic field to which the associated labelis exposed is at a first power level, said first mode being that of alinear circuit with an impedance dependent upon frequency, and the oreach label circuit is adapted to operate in a second mode when theelectromagnetic field to which the associated label is exposed is at asecond power level, said second mode being that of a non-linear circuitwith behaviour dependent upon time.
 2. A system as claimed in claim 1wherein said presence detection means includes a presence detectionsignal generator, a presence detection antenna for creating saidpresence detection electromagnetic field, and an analyser of impedanceassociated with said presence detection antenna.
 3. A system as claimedin claim 1 wherein said data communications means includes a generatorof an interrogation signal, an interrogation antenna for creating saidinterrogation electromagnetic field, a data receiver antenna, and areceiver and decoder of data communication signals.
 4. A system asclaimed in claim 1, wherein the same label circuit is used for bothpresence detection and data communications.
 5. A system as claimed inclaim 4 including means for modifying presence indication behaviour ofthe or each coded label upon which presence detection is based.
 6. Asystem as claimed in claim 5 wherein the presence indicationmodification is accomplished by electromagnetic means.
 7. A system asclaimed in claim 5 wherein the presence indication modification isirreversible.
 8. A system as claimed in claim 5, wherein the presenceindication modification includes a change in an impedance within theassociated label circuit.
 9. A system as claimed in claim 5, wherein thepresence indication modification includes a change in a conducting pathwithin the associated label circuit.
 10. A system as claimed in claim 9wherein the presence indication modification introduces either an opencircuit it or a short circuit.
 11. A system as claimed in claim 1wherein said interrogation electromagnetic field is capable of changingdata within the or each coded label.
 12. A system as claimed in claim 5wherein data within the or each coded label is changed when its presenceindication behaviour is modified.
 13. A system as claimed in claim 5wherein data within the or each coded label is read to confirm that themodification in presence indication behaviour is legitimate.
 14. Asystem as claimed in claim 1 wherein the same label antenna is used forsignalling label presence and for data communications.
 15. A system asclaimed in claim 1 wherein the same frequency range is used for presencedetection and for data communications.
 16. A system as claimed in claim1 wherein the label antenna used for presence detection and the labelantenna used for data communications are shaped so as to achieve maximumrange subject to electromagnetic compatibility regulations.
 17. A systemas claimed in claim 5, wherein the presence indication modification isaccomplished without damaging the data communications function.
 18. Asystem as claimed in claim 1 wherein the or each label circuit includesa resonant circuit used by the presence detection means and a resonantcircuit used by the data communications means.
 19. A system as claimedin claim 18 wherein in the same resonant circuit is used by the presencedetection means and the data communications means.
 20. A system asclaimed in claim 19 wherein the frequency of the resonant circuit isdependent upon the amplitude of the electromagnetic field to which theassociated label is exposed.
 21. A system as claimed in claim 18 whereinthe frequency of the resonant circuit used by the presence detectionmeans is different from the frequency of the resonant circuit used bythe data communications means.
 22. A system as claimed in claim 1wherein the presence detection means includes a plurality of antennas.23. A system as claimed in claim 22 wherein at least a pair of theplurality of antennas are uncoupled when the coded label is absent. 24.A system as claimed in claim 18 wherein the quality factor of theresonant circuit used by the presence detection means is different fromthe quality factor of the resonant circuit used by the datacommunications means.
 25. A system as claimed in claim 24 wherein thequality factor of the resonant circuit used by the presence detectionmeans is as high as is practicable, while the quality factor of theresonant circuit used by the data communications means is adjusted toprovide a bandwidth sufficient to contain the signals used by the datacommunications means.
 26. A system as claimed in claim 1 wherein the oreach coded label includes a controller, a modulator and a time delaycircuit, such that the associated label circuit does not operate in saidsecond mode until a time delay determined by said time delay circuit hasexpired.
 27. A system as claimed in claim 5 wherein the presenceindication modification is accomplished by removal of a capacitor fromthe label circuit.
 28. A system as claimed in claim 27 wherein saidremoval of a capacitor from the label circuit is performed by a punchingoperation.
 29. A system as claimed in claim 1 wherein the or each labelcircuit includes a series resonant circuit.
 30. A system as claimed inclaim 29 wherein the or each label circuit includes two or more mutuallyuncoupled inductors.
 31. A system as claimed in claim 1 wherein the oreach label circuit includes a voltage limiting device.
 32. A system asclaimed in claim 2 wherein the presence detection means explores theimpedance associated with the presence detecting antenna at a discreteset of frequencies.
 33. A system as claimed in claim 32 wherein thediscrete set of frequencies form a pseudo random sequence.
 34. A systemas claimed in claim 2 wherein signals reaching the analyser of thepresence detection means are ignored while the presence detection signalgenerator is inactive.
 35. A system as claimed in claim 1 wherein theelectromagnetic field created by the presence detection means or thedata communications means is of octopole form.
 36. A system as claimedin claim 2 wherein the presence detection signal generator includes afrequency selectable oscillator.
 37. A system as claimed in claim 36wherein said oscillator operates at a plurality of discrete frequencies.38. A system as claimed in claim 37 wherein the plurality of discretefrequencies forms a pseudo random sequence.
 39. A system as claimed inclaim 36, wherein the or each coded label includes a tuned circuit forindicating label presence and the time for which a particular frequencyof the frequency selectable oscillator is generated is small in relationto the rise time of a detection section of a CISPR quasi peak detector,but long in relation to the rise time of the tuned circuit.
 40. A systemas claimed in claim 1 wherein said presence detection means includes apresence detection antenna, a microcontroller and an adjustableimpedance, said impedance being adjusted so that variations in itsmagnitude and phase with respect to frequency closely matchescorresponding variations with respect to frequency of the impedance ofthe presence detection antenna.
 41. A system as claimed in claim 40including signal separation means, wherein the impedance is adjusted tominimise mean squared error of an unbalanced signal emerging from thesignal separation means.
 42. A system as claimed in claim 3 wherein theinterrogation signal generator includes a frequency selectableoscillator.
 43. A system as claimed in claim 42 wherein said oscillatoris switched to produce a range of discrete frequencies.
 44. A system asclaimed in claim 43 wherein said frequencies form a pseudo randomsequence.
 45. A system as claimed in claim 42, wherein the time forwhich a particular frequency of said frequency selectable oscillator isgenerated is short compared with the rise time of a CISPR quasi peakdetector, but is long in relation to the rise time of the tuned circuitwithin the data communications antenna when the label is operating inits data communication mode.
 46. A system as claimed in claim 45 whereinthe or each coded label generates a sub-carrier frequency and the timefor which a particular frequency of the frequency selectable oscillatoris generated is short in relation to the period of the sub-carrierfrequency.
 47. A system as claimed in claim 4 wherein said label circuitconsists of the same elements interconnected in the same way.
 48. Asystem as claimed in claim 1 wherein when the electromagnetic field towhich the associated label is exposed is between said first and secondpower levels, the or each label circuit is adapted to operate in a thirdmode which is a substantial superposition of said first and secondmodes.
 49. A system as claimed in claim 1 wherein the or each presencesignalling antenna includes a magnetic field responding antenna.
 50. Asystem as claimed in claim 1 wherein the first and second modes areperformed by respective components in the associated label circuit whichare separated.
 51. A system as claimed in claim 1 wherein the or eachlabel circuit includes a first capacitor having a relatively lowbreakdown voltage and a second capacitor which is relatively robust andwherein said first capacitor is significantly smaller than said secondcapacitor.
 52. An electronic coded label for use with a system fordetecting presence thereof and for communicating therewith, said labelincluding a presence signalling antenna, a label circuit and a datacommunications antenna, wherein said label circuit is adapted to operatein a first mode when an electromagnetic field to which said label isexposed by said system is at a first power level, said first mode beingthat of a linear circuit with an impedance dependent upon frequency, andsaid label circuit is adapted to operate in a second mode when theelectromagnetic field to which said label is exposed by said system isat a second power level, said second mode being that of a non-linearcircuit with behaviour dependent upon time.